Geometric approximation on proximal normals. Radulescu, M.L.; Clarke, F.H. — 1997 Journal of Convex Analysis
Proximal smoothness and the lower- C 2 property. Clarke, F.H.; Stern, R.J.; Wolenski, P.R. — 1995 Journal of Convex Analysis
Complements, approximations, smoothings and invariance properties. Clarke, F.H.; Ledyaev, Yu.S.; Stern, R.J. — 1997 Journal of Convex Analysis